The present invention relates generally to a convergence measuring apparatus for measuring the convergence state of a color CRT of a television receiver or the like.
As is well known, in color CRTs uniform convergence of primary colors in the CRT is desirable to provide a high quality image display. One aspect of quality control in the manufacture of CRTs is convergence measurement.
Previously proposed convergence measuring devices are exemplified by a phase detection type convergence measuring device such as disclosed in Japanese Patent Application No. 63-310670, by the same applicant as the present invention. In these prior proposed apparatus, a pattern generator outputs image signals to a color CRT to be measured. The pattern generator produces image signals by which a luminescent line (horizontal or vertical) of a primary color is shifted on one of the domains of the viewing area of the CRT with a white area being cast on another domain. An image sensor having a single humped directional sensitivity is positioned on the face of the CRT.
With this arrangement, the light sensor is positioned at an arbitrary position on the viewing screen of the CRT and a luminescent line of each primary color is cast on the viewing screen by the pattern generator. A computer then creates an envelope curve for each of the primary colors in accordance with a detection output, corresponding to each of the primary colors, from the light sensor so as to obtain the position of the peak value of each envelope curve, and compares the positions of the peak values of the primary colors with each other so as to calculate a mis-convergence value. Further, the position of the light sensor is changed on the viewing screen of the CRT so that the measurement is carried out at several positions thereon.
The reason for providing a white domain during the above-described measurement is, if the respective luminescent lines of green, red and blue are solely cast on the viewing screen the beam current of the CRT changes, causing high-voltage variation. Such variation causes a positional shift of an electronic peak for each color, and accordingly the white domain is provided to stabilize high voltage variations.
However, a number of problems exist with this system of measurement. First of all, even though the white domain is provided as mentioned above, since the current value of a luminescent line of the presently displayed color is greater than that of the other colors, the current values of the RGB beams vary each time the color of the luminescent line is changed. High voltage variations caused by the change in current value, by way of a voltage drop, cause a positional shift of the luminescent line which gives rise to measurement error.
In order to prevent the above occurrence it is generally understood that the width of the displayed primary color lines can be reduced to minimize the difference between the current values of the colors. But, if the width of the luminescent line is narrowed, disadvantages such as deterioration of the signal to noise (S/N) ratio or the like would occur. Accordingly this is not a viable way of solving the problem.
Further, with the above-described type of measuring apparatus although the convergence condition of a color CRT can easily be measured, unless the measuring apparatus itself is operating properly, precise measurement results cannot, of course, be expected. Thus, checking has conventionally been done by using a `control` CRT or a dummy signal generator. The former involves the use of a color CRT prepared so as to have a known mis-convergence value. This known data is then compared with measurement data in order to check the measurement apparatus. In the latter case a dummy signal simulating a known luminescence of a color CRT is generated from the dummy signal generator and the dummy signal is used in place of an output from the light sensor to be input to the measuring apparatus so as to check its accuracy.
The drawbacks of these methods are that they both require the use of bulky extra equipment, such as a standard CRT or a dummy signal generator. As a result, such checking is difficult to implement smoothly during use of the measuring equipment.